In Piezo Response Force Microscopy (PFM) thin films of piezoelectric materials are investigated by means of a scanning probe microscope. An AC voltage is applied between the probe and the sample in order to generate an alternating electric field in the sample in order to obtain the piezoelectric response.
The probing tip touches the surface with a controlled normal DC force. The oscillation of the tip due to the piezo mechanical response of the sample caused by the AC signal is detected with a lock-in amplifier and is a measure of the polarization of the piezo electric sample.
This type of measurement is e.g. widely used for characterizing thin ferroelectric storage media. In particular, it can be applied to obtain a spatially resolved image of the polarization direction and the domains of the sample.
In order to enhance the sensitivity, one would like to use the mechanical resonance of the cantilever. But the resonance frequency is changing when moving the tip along the surface. To stay on resonance, one would like to use a PLL or a self excitation loop. But this is not possible due to the phase changes by 180 degrees when moving from one polarization to a reversed polarization of the sample. When passing regions with no polarization, the oscillation cannot be maintained via excitation of the piezoelectric material. Several Solutions have been proposed for this problem:
Dual Frequency Resonance Tracking (DFRT): the resonance is excited at two frequencies, one slightly above and one slightly below the resonance frequency. The response amplitudes at these two frequencies, A1 and A2, as measured with two separate lock-ins. The difference A1−A2 is a measure of the drift of the resonance frequency. A feedback loop can readjust the resonance frequency and the two excitation frequencies. The disadvantage of this techniques is that the low-pass filters of the lock-ins have to be narrow such that the two frequencies do not disturb each other. For high Q resonances this results in very slow measurements.
Band excitation: Basically a complete frequency spectrum is recorded at each measurement point. This is done by generating a special excitation waveform which contains all frequencies within the band where the resonance frequency is supposed to be. The response is then processed with a Fourier transform, and the amplitude, resonance frequency, and the quality factor are determined. The disadvantage of this technique is that it requires high processing power and that it is quite slow since some time has to be spent on each measurement point.
WO 2008/071013 is in many aspects similar to the present invention. However, it is directed to the measurement of electrostatic interactions between sample and probe, and not the piezoelectric response of the probe. For reasons explained at the end of the description, this prior art uses similar technologies as the present invention, but applies them in a very different manner.
Other related prior art: Nanotechnology 18(2007) 435503 (8pp): The band excitation method in scanning probe microscopy for rapid mapping of energy dissipation on the nanoscale. Stephen Jesse, Sergei V Kalinin, Roger Proksch, A P Baddorf, and B J Rodriguez.